1. Field of the Invention
The present invention relates to an amplifier having a cascade circuit, more particularly to a structure for keeping down the signal voltage generated at the node of the two transistors connected in cascade.
An amplifier circuit such as an operational amplifier is usually operated by using a negative feedback circuit. When the input signal has a high frequency, however, the negative feedback easily turns to positive feedback due to floating capacitances and so forth, so that the output undesirably oscillates. Therefore, some means of preventing oscillation at high frequency input signals is required.
Especially, operational amplifiers are very often used in an analog circuit. Therefore, the operating speed of the analog circuit is determined mainly by the operating speed of the operational amplifiers. When the operational amplifiers are constructed by metal oxide semiconductor (MOS) transistors, the driving performance of he output-stage transistor is lowered at a high frequency range because the nutual conductance g.sub.m of the MOS transistor is lower by one or two orders than that of the bipolar transistor when the sizes of these transistors are the same. The lower driving performance also causes the oeprational amplifier to be turned to a positive feedback, and the positive feedback causes an oscillation of the output, resulting in a lowered operating speed or an unstable operation. Thus, a wide band, stable and high speed operational amplifier is required.
2. Description of the Related Art
An operational amplifier is often constructed by a differential amplifier stage and an output stage. The differential amplifier stage is usually constructed by cascade circuits each having two MOS transistors connected in series.
In a conventional cascade circuit, the gate electrode of one of the two transistors is connected to a fixed potential source, to suppress the amplitude at the connecting point between the two transistors. Further, to prevent an oscillation at the output of the operational amplifier, a phase compensating capacitor is connected between the above-mentioned connecting point nd the output end, to lower the gain of the amplifier at a high frequency range as disclosed in, for example, Japanese Unexamined Patent Publication No. 59-43613 (Hitachi), and as described later in more detail with reference to the drawings.
The above-mentioned conventional amplifier, however, still has a disadvantage of oscillation at a high frequency range. That is, even though the cascade circuits of the prior art enable the sinal amplitude of the connecting point to be kept low, the suppression of the signal amplitude is not sufficient and a further improvement is necessary.